There are many engineering challenges in the aircraft PM emission measurement field: it is difficult to dilute aircraft exhaust at the probe tip; the exhaust pressure at the engine exit varies tremendously from idle to maximum power, which makes it extremely difficult for any one sampling system to provide adequate dilution and deliver samples with stable pressure downstream; and due to the harsh testing environment, the sampling line is normally long (up to 140 ft) and no inline pump is allowed for PM measurement. A vacuum pump at the end of the sample line to aid in sample transfer would result in a sample at sub-ambient pressure which could lead to PM measurement instrument malfunctioning. To overcome these difficulties, PM emission testing normally demands a large effort in test setup which translates into a high cost.
The Aircraft Environmental Support Office (AESO) of the US Navy has previously used the EPA Method 5 for aircraft PM emission testing. This method used a large probe to capture exhaust samples for gravimetric analysis. This method is time consuming (high cost) and has a high detection limit that is not suitable for testing the low emission modern engines.
New methods have been developed within the aircraft PM emission measurement community in the last decade. To preserve the PM properties emitted from an aircraft engine, probe tip dilution is recommended (SAE-AIR 6037), which make it extremely difficult and costly to perform the PM emission measurement. Currently, it is not clear how much the PM properties will be affected when the sample is diluted downstream instead of at the probe tip.
For practical purposes, it is necessary to evaluate whether downstream dilution can be used to yield similar emission data as probe tip dilution. It is desirable to establish a sampling system that can function smoothly under any power condition and be able to deliver exhaust samples through a long sampling line at a pressure level within the operating limits of PM measurement instruments.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings and in the appended claims.